"Over the past twenty years" can both refer to something that has happened during the past twenty years. It could happen all the time or it might happen now and then.
"Over the past twenty years, pirate attacks have increased off the coast of Somalia."
You could also start out "Since 20 years ago."
If I was talking about something that happened 20 years ago and affects things now, I would only say "since 20 years ago."
"The law has been in effect since 20 years ago."
Though, more often you would hear it as
"The law has been in effect for 20 years."